- Remove old comment for PURGE() as it no longer exists and implied it   was a comment to cache_zap(). - Add a comment to quickly state what cache_zap() does.
- Remove old comment for PURGE() as it no longer exists and implied it
  was a comment to cache_zap().
- Add a comment to quickly state what cache_zap() does.

Reviewed by:	phk, mux

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase